Cytoplasmic inclusions of the TAR DNA-binding protein of 43 kDa (TDP-43) is frequently found in degenerating neurons of patients with Amyotrophic lateral sclerosis, Frontotemporal Dementia, and Alzheimer?s Disease. TDP-43 is a ubiquitously-expressed, tightly-regulated, and predominantly nuclear DNA/RNA-binding protein and the cause and mechanism underlying the formation of TDP-43 protein inclusions remains unclear. Furthermore, modeling these inclusions has proven challenging and it is unclear if these are neurotoxic or neuroprotective. To address these issues, we developed an optogenetic approach to induce TDP-43 inclusions in live cells. We show that these inclusions mimic the properties of TDP-43 inclusions found in patient neurons that are neurotoxic. We further show that RNA binding status dictates the propensity for TDP-43 for form inclusions in live cells. In this grant, we propose to identify the characteristics of neurotoxic TDP-43 assemblies employing our light-induced system. Specifically, we will describe the post- translational modifications, localization, and material state of neurotoxic TDP-43 assemblies. Since our preliminary data indicates that RNA binding state determines whether TDP-43 proteins can oligomerize into a pathogenic conformation, we propose to study whether bait oligonucleotides designed to bind RNA deficient TDP-43 proteins confer neuroprotection in cortical neuron cultures, and in vivo using a TDP-43 Drosophila model system developed in our labs. These studies have the potential to address pervasive questions regarding the neurotoxic state of TDP-43 assemblies and the development of a new use for oligonucleotide therapeutics.